A. Field of Invention
The present invention relates to compositions for forming thermoplastic vulcanizates and methods of forming thermoplastic vulcanizates to improve extrusion properties.
B. Description of the Related Art
Thermoplastic vulcanizates (“TPV”) are vulcanized compositions that include a large amount of finely dispersed crosslinked elastomer particles in a continuous thermoplastic. TPVs are vulcanized by a process called dynamic vulcanization—a process of selectively crosslinking the elastomer component during its melt mixing with the molten thermoplastic. TPVs have the benefit of the elastomeric properties provided by the elastomer phase, with the processability of thermoplastics.
TPVs are routinely used in the manufacture of a variety of products, the form of which is created by means of extruding the molten TPV through a die, wherein the die is shaped to impart the desired form to the extrusion. One common problem that arises in this extrusion process is that as the TPV material passes through the die, material deposits form on the outer lip of the die. The deposits form what are commonly referred to as a “moustache” on the die lip, or “die-lip buildup”.
While the specific causes of die lip buildup during the TPV extrusion process are widely discussed and disputed, it is undisputed that die lip buildup poses a significant problem for extruders. An effort to describe the causes of die lip buildup in conventional thermoplastic extrusion is made by Jessie D. Gander and A. Jeffrey Giacomin in “Review of Die Lip Buildup in thermoplastics Extrusion” 37(7)POLYMER ENGINEERING AND SCIENCE (July-1997). Though this reference refers to thermoplastics extrusion, it is believed to be an apt reference with respect to the cause of die lip buildup in thermoplastic vulcanizate extrusion. The shape, and more specifically, the consistency of the shape, imparted to the molten TPV as it passes through the extruder die is dependant on the smoothness of the die at its mouth. The presence of material deposits or buildup at the mouth of the extruder die can affect the shape of the die mouth. Additionally, deposits may impart unwanted grooves on the extrusion as the deposit digs into the surface of the molten extrusion. Finally, deposits may, from time to time, slough off of the die lip and onto the TPV extrusion, creating an unwanted bead or glob on the extrusion.
As a result of die lip buildup, it may be necessary to periodically shut down the extrusion line in order to manually remove the deposits. This is highly undesirable. Alternatively, or additionally, it may be necessary to discard portions of the extrusion that have been marred by the buildup. This can result in material waste, increased manufacturing time, increased cost, and lower overall quality.
Methods to reduce die lip buildup have generally produced unsatisfactory results, either in the total reduction of die lip buildup, or in the overall physical properties of the resultant TPV. Two approaches are described in EP 0 911 364 and EP 1 550 693. In the first reference, a process for making a TPV is described that involves adding a portion of the total polyolefin thermoplastic to the TPV composition after the elastomer curing agent has been added to the extrusion chamber and the elastomer has been substantially cured. This approach to reducing die lip deposits would appear to be predicated on the theory that protecting a portion of the thermoplastic from degradation by the peroxide curing agent, by adding a portion of the thermoplastic after the curing agent has been depleted in vulcanizing the constituent elastomer, will result in improved TPV physical characteristics that lead to reduced buildup on later extrusion. However, the reduction resulting from this method is not satisfactory. In the latter reference (EP 1 550 693), syndiotactic polypropylene is substituted for isotactic polypropylene as the preferred polyolefin thermoplastic. While reduced die buildup is claimed, the resultant TPV has poor compression data at the extrusion temperature.
It would be desirable to provide TPV compositions and methods of forming thermoplastic vulcanizates that result in a reduction of die lip buildup as compared to conventionally formed TPVs while maintaining other physical properties.